Instrument Loop Diagrams: Preparation, Types, Symbols and Examples

What is an Instrument loop diagram?

An Instrument Loop Diagram (ILD) is an engineering drawing that shows the electrical connections, signal paths, terminal details, and associated components of an individual instrumentation control loop. It provides detailed information about how a field instrument is connected to a junction box, marshalling cabinet, control system, and other devices within the loop.

Instrument loop diagrams are widely used in process plants, refineries, petrochemical facilities, power stations, and industrial automation projects. They help instrumentation engineers verify field wiring, prepare installation activities, perform loop checking, and troubleshoot signal transmission problems.

A typical instrument loop diagram includes the instrument tag number, terminal identification, cable number, junction box terminals, marshalling cabinet details, barriers or isolators where applicable, and PLC or DCS input and output channel information.

Unlike a Piping and Instrumentation Diagram (P&ID), which explains the process arrangement and instrumentation functions, an instrument loop diagram focuses on the detailed electrical and signal connections of a specific loop.

This guide explains the purpose, types, preparation, symbols, interpretation, and practical applications of instrument loop diagrams. It also covers the engineering documents required to prepare an ILD and the checks required before construction and commissioning.

Detailed Explanation of Instrument Loop Diagrams:

Each process control loop has an instrument loop diagram (ILD) developed for it. ILDs tell you more about control loops than any other drawing. The ILD is a detailed drawing that shows how field instruments like transmitters, switches, actuators and control valves etc., are connected to the control system. It could be

• Signals between the Control system for field instruments
• Signals between the Control Panel and the control system
• Signal between the MCC and  control system
• Signal between one control system to another

The loop diagram shows the instrument (in a symbol) and its terminal numbers that need to be connected, the cable number, the junction box number, the terminal number assigned to the specified instrument, the multipair cable and pair number, the marshalling cabinet number, the terminal number in the marshalling cabinet, and the control system details (rack, slot, I/O channel).

Difference Between Instrument Loop Diagram, P&ID, Wiring Diagram and Loop Folder

Engineering document	Main purpose	Information provided
P&ID	Shows process arrangement and control functions	Equipment, pipelines, valves, instrument tags, and process connections
Instrument Loop Diagram	Shows individual instrument loop connections	Field terminals, cables, junction boxes, marshalling, barriers, and control system channels
Wiring Diagram	Shows electrical wiring and terminal connections	Wire numbers, terminal numbers, connections, and electrical details
Instrument Hook Up Drawing	Shows physical instrument installation	Mounting brackets, fittings, tubing, manifolds, and installation arrangement
Instrument Loop Folder	Contains the complete documentation package for a loop	Loop diagram, datasheet, calibration records, installation records, and commissioning results

Important engineering note: An instrument loop diagram is one document within an instrument loop folder. The loop folder is not another name for the loop diagram. It contains the loop diagram and other relevant engineering and commissioning records.

What sources are required to make a loop diagram?

To finish the loop diagram and give full information, the following information is needed, along with its source or reference:

Instrument tag number with terminal number:

It’s likely that most instruments use (+) and (-). terminals. Instruments that need to be set up in a certain way, like smoke detectors or instruments in a series loop, need connection details from the manufacturer to make sure the cable is properly connected.

Junction box terminal number:

You could get this information from the JB wiring connection.

Terminals number in marshalling cabinet:

This information could be found by marshalling drawing wiring connections.

I/O point detail information:

Get this information from the I/O assignment that the system integrator or control system vendor makes.

Purpose of Instrument Loop Diagrams

ILD is used to check that the installation and connections are done correctly during pre-commissioning, commissioning, and operation. It can also be used to troubleshoot the problems.

How to read/interpret a loop diagram?

Below we can see an example of an instrument loop diagram with an analogue input & output from DCS along with digital output from ESD systems:

The Instrument loop drawing shown above have two sections field side (process area) and system side (Marshalling & System cabinet and HMI).

1. Field Side:

The field side is divided into two sections. The first is an instrument, whereas the second is a junction box where cables link.

Instrument:

FIT01 represents a Flow Indicator cum transmitter and FV01 indicates a Flow control valve

Junction Box:

JBA-001 and JBSV-001 represents two different junction boxes.

Where, JBA-001 junction box is connected to FIT01 flow transmitter and FY01 flow control valve on the field side, and DCS system on the system side.

On the other hand, JBSV-001 junction box is connected to XY01 actuator on the field side, and ESD system on the system side.

2. System Side:

The System side is further divided into three sections. The marshalling cabinet, System cabinet and HMI.

Marshalling cabinet:

JBA-001 junction box is connected to MAR-DCS-01 DCS Marshalling cabinet through TBA-001 terminal blocks.

On the other hand, JBSV-001 junction box is connected to MAR-ESD-01 ESD Marshalling cabinet through TBSV-001 terminal blocks.

System cabinet:

MAR-DCS-01 Marshalling cabinet’s flow transmitter signal (FIT01) is connected to SYS-DCS-01 system cabinet through FIT-01 signal isolator (Barrier) to Analog input module.

Controller: 01

Rack: 2

Slot/Module: 5

Channel No: 3

IO Type: AI

MAR-DCS-01 Marshalling cabinet’s flow control valve signal (FY01) is connected to SYS-DCS-01 system cabinet through FY-01 signal isolator (Barrier) to Analog output module.

Controller: 01

Rack: 2

Slot/Module: 6

Channel No: 3

IO Type: AO

MAR-ESD-01 Marshalling cabinet’s actuator signal (XY01) is connected to SYS-ESD-01 system cabinet through XV-01 signal isolator (Barrier) to Digital output module.

Controller: 01

Rack: 1

Slot/Module: 3

Channel No: 3

IO Type: DO

HMI:

Finally the flow transmitter (FIT01) and flow control valve (FY01) signals are connected to FIC01 and FY01 Analog input and output data points in DCS system and configured the same in Human Machine Interface (HMI) display for monitoring and control purpose.

Various types of Loop diagrams:

1. A PCS/DCS loop diagram can be used to show the connection details of field instruments with DCS system.
2. Emergency Shutdown (ESD) loop diagram can be used to show the connection details of Safety Instrumented System (SIS) field instruments with ESD system.
3. Fire & Gas (F&G) loop diagram can be used to show the connection details of F&G related field instruments with F&G system.
4. System Interconnection loop diagram can be used to show the connection details between one control systems to another.

Few of the examples of System Interconnection loop diagram is mentioned below

a) Hardwired signals between DCS and ESD system for tripping purpose.

b) Hardwired signals between DCS and F&G system for alarming purpose.

c) Hardwired signals between DCS and Motor Control room (MCC) system for motor/pump start and stop signals.

d) Hardwired signals between DCS and Interposing Relay Panel (IRP) for motor/pump start and stop signals.

Some Useful Questions related to Loop drawings:

What is loop number of an instrument?

A loop number is an unique number assigned to a specific control loop within a system to identify the control loop.

A control loop is a closed-loop control system that is used to maintain a process variable at a desired setpoint by continuously measuring and adjusting the output of a control system. The loop number may be used to identify and distinguish different control loops within a system.

For example, a manufacturing facility may have multiple control loops to maintain different process variables such as temperature, pressure, flow rate, and so on. Each of these control loops may have a unique loop number to identify it within the system.

The loop number may be used to access and configure the settings of a particular control loop, or to monitor and troubleshoot issues with the control loop.

What is an instrument loop folder?

An instrument loop folder is a document that provides detailed information about a specific control loop within a system. The loop folder typically includes a variety of information, such as the process variables being controlled, the control system components, the control strategies and algorithms being used, and the instrumentation and sensors that are used to measure the process variables.

The loop folder is typically used as a reference document for engineers, technicians, and other personnel who need to understand the details of a particular control loop in order to maintain, troubleshoot, or modify the system. It may also be used as a training resource for new personnel who are learning about the control loop.

In addition to providing detailed information about the control loop, the loop folder may also include diagrams, schematics, and other visual aids to help explain the system and its components. It may also include information about the process being controlled, such as the process flow, the materials being used, and the operating conditions.

How to Prepare an Instrument Loop Diagram

Preparation of an instrument loop diagram requires coordination between instrumentation engineering, electrical engineering, control system engineering, and system vendors.

Step 1: Identify the Instrument Tag Number

Refer to the P&ID and instrument index to identify the correct instrument tag number, service description, and control loop.

Step 2: Collect Engineering Documents

Collect the approved instrument datasheet, cable schedule, junction box schedule, hook up drawing, I/O list, marshalling drawing, and manufacturer wiring information.

Step 3: Identify Field Instrument Connections

Check the manufacturer’s terminal diagram and identify the required terminals, power connections, signal terminals, grounding points, and communication connections.

Step 4: Identify Cable and Junction Box Connections

Refer to the cable schedule and junction box drawings to establish the field cable number, cable cores or pairs, and junction box terminal assignments.

Step 5: Develop the Marshalling Connections

Identify the marshalling cabinet, terminal blocks, interposing components, barriers, and isolators where applicable.

Step 6: Identify Control System I/O Connections

Confirm the PLC or DCS cabinet, rack, slot, channel, I/O card type, signal type, and control system tag.

Step 7: Prepare the Loop Drawing

Arrange the field instrument, cable, junction box, marshalling, and control system information in a clear drawing format.

Include terminal numbers, cable identification, wire connections, signal details, grounding, and applicable notes.

Step 8: Perform Engineering Verification

Check the drawing against the approved source documents. Confirm tag numbers, terminal assignments, cable numbers, I/O channels, signal types, and interface details.

Step 9: Issue the Drawing for Approval

Submit the drawing for the required engineering review and client approval. Incorporate comments and issue the approved revision for construction.